This application claims priority under 35 USC 119 of Japanese Patent Application No. 2000-63337 filed on Mar. 3, 2000, the entire disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an engine overrun prevention system for an automatic transmission, and more particularly to such system suitable for a large vehicle.
2. Description of the Related Art
Recently, in order to lighten a driver""s burden, large vehicles such as tractors and large tracks are often equipped with an automatic clutch and automatic transmission. In such vehicles, a target transmission gear is determined by a vehicle speed or driver""s moving a shift lever, a clutch is automatically disengaged and engaged and the transmission is automatically shifted to a desired gear.
When a vehicle is running on a down slope with an engine brake, a driver sometimes shifts the transmission to a lower gear in order to enhance the engine brake. In this situation, when the clutch is disengaged, the vehicle runs without any load. Therefore, the vehicle is accelerated until the clutch is engaged. If the steep shifting down occurs, e.g., from the 8th gear to the 3rd gear, the engine is caused to rotate at an excessively high speed (i.e., overrun) upon engaging of the clutch. This results in an excessive deceleration shock and makes the driver feel uncomfortable.
In particular, the tractor has a greater number of gear positions, e.g., 16 gear positions, than a normal passenger car, and gear ratios are close to each other on a low gear side. Accordingly, the driver tends to shift the transmission to a very low gear. This causes, as mentioned above, overrunning of the engine and excessive deceleration shock.
An object of the present invention is to overcome the above described problems.
According to one aspect of the present invention, there is provided an engine overrun prevention apparatus suitable for an automatic transmission including a first controller for calculating an engine revolution speed of after shifting to a target gear position from a current gear position, based on vehicle speed and a gear ratio of an automatic transmission at the target gear position, and a second controller for changing the target gear position to a higher gear position when the calculated engine revolution speed is equal to or greater than a preset value. Since the target gear position is raised when an engine revolution speed is assumed to become greater than the predetermined value, the shifting down is moderated. As a result, an engine overrun upon shifting down will not occur. If the target gear position is one gear below a current gear position, then no shifting down is admitted as far as the engine overrun is expected.
The second controller may change the target gear position by one or more gears. The target gear position may be determined from manual movement of a shift lever. The automatic transmission may be a remote control type manual transmission that is shifted up and down under the control of another controller based on a signal generated upon manual movement of the shift lever. This signal may be generated by a shift switch associated with the shift lever.
According to another aspect of the present invention, there is provided an engine overrun prevention method including the steps of determining whether an engine revolution speed at a target gear position is greater than a preset value, and changing the target gear position to a higher gear position.
Additional objects, benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the embodiments and the appended claims, taken in conjunction with the accompanying drawings.